Basic Hooks

useState

During the initial render, the returned state (state) is the same as the value passed as the first argument (initialState).

The setState function is used to update the state. It accepts a new state value and enqueues a re-render of the component.

setState(newState);

During subsequent re-renders, the first value returned by useState will always be the most recent state after applying updates.

Functional updates

If the new state is computed using the previous state, you can pass a function to setState. The function will receive the previous value, and return an updated value. Here’s an example of a counter component that uses both forms of setState:

The ”+” and ”-” buttons use the functional form, because the updated value is based on the previous value. But the “Reset” button uses the normal form, because it always sets the count back to 0.

Note

Unlike the setState method found in class components, useState does not automatically merge update objects. You can replicate this behavior by combining the function updater form with object spread syntax:

setState(prevState =>{// Object.assign would also workreturn{...prevState,...updatedValues};});

Another option is useReducer, which is more suited for managing state objects that contain multiple sub-values.

Lazy initialization

The initialState argument is the state used during the initial render. In subsequent renders, it is disregarded. If the initial state is the result of an expensive computation, you may provide a function instead, which will be executed only on the initial render:

useEffect

useEffect(didUpdate);

Accepts a function that contains imperative, possibly effectful code.

Mutations, subscriptions, timers, logging, and other side effects are not allowed inside the main body of a function component (referred to as React’s render phase). Doing so will lead to confusing bugs and inconsistencies in the UI.

Instead, use useEffect. The function passed to useEffect will run after the render is committed to the screen. Think of effects as an escape hatch from React’s purely functional world into the imperative world.

Cleaning up an effect

Often, effects create resources that need to be cleaned up before the component leaves the screen, such as a subscription or timer ID. To do this, the function passed to useEffect may return a clean-up function. For example, to create a subscription:

The clean-up function runs before the component is removed from the UI to prevent memory leaks. Additionally, if a component renders multiple times (as they typically do), the previous effect is cleaned up before executing the next effect. In our example, this means a new subscription is created on every update. To avoid firing an effect on every update, refer to the next section.

Timing of effects

Unlike componentDidMount and componentDidUpdate, the function passed to useEffect fires after layout and paint, during a deferred event. This makes it suitable for the many common side effects, like setting up subscriptions and event handlers, because most types of work shouldn’t block the browser from updating the screen.

However, not all effects can be deferred. For example, a DOM mutation that is visible to the user must fire synchronously before the next paint so that the user does not perceive a visual inconsistency. (The distinction is conceptually similar to passive versus active event listeners.) For these types of effects, React provides one additional Hook called useLayoutEffect. It has the same signature as useEffect, and only differs in when it is fired.

Although useEffect is deferred until after the browser has painted, it’s guaranteed to fire before any new renders. React will always flush a previous render’s effects before starting a new update.

Conditionally firing an effect

The default behavior for effects is to fire the effect after every completed render. That way an effect is always recreated if one of its inputs changes.

However, this may be overkill in some cases, like the subscription example from the previous section. We don’t need to create a new subscription on every update, only if the source props has changed.

To implement this, pass a second argument to useEffect that is the array of values that the effect depends on. Our updated example now looks like this:

Now the subscription will only be recreated when props.source changes.

Passing in an empty array [] of inputs tells React that your effect doesn’t depend on any values from the component, so that effect would run only on mount and clean up on unmount; it won’t run on updates.

Note

The array of inputs is not passed as arguments to the effect function. Conceptually, though, that’s what they represent: every value referenced inside the effect function should also appear in the inputs array. In the future, a sufficiently advanced compiler could create this array automatically.

useContext

const context =useContext(Context);

Accepts a context object (the value returned from React.createContext) and returns the current context value, as given by the nearest context provider for the given context.

When the provider updates, this Hook will trigger a rerender with the latest context value.

Additional Hooks

The following Hooks are either variants of the basic ones from the previous section, or only needed for specific edge cases. Don’t stress about learning them up front.

useReducer

const[state, dispatch]=useReducer(reducer, initialState);

An alternative to useState. Accepts a reducer of type (state, action) => newState, and returns the current state paired with a dispatch method. (If you’re familiar with Redux, you already know how this works.)

Here’s the counter example from the useState section, rewritten to use a reducer:

Lazy initialization

useReducer accepts an optional third argument, initialAction. If provided, the initial action is applied during the initial render. This is useful for computing an initial state that includes values passed via props:

useReducer is usually preferable to useState when you have complex state logic that involves multiple sub-values. It also lets you optimize performance for components that trigger deep updates because you can pass dispatch down instead of callbacks.

useCallback

Pass an inline callback and an array of inputs. useCallback will return a memoized version of the callback that only changes if one of the inputs has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders (e.g. shouldComponentUpdate).

useCallback(fn, inputs) is equivalent to useMemo(() => fn, inputs).

Note

The array of inputs is not passed as arguments to the callback. Conceptually, though, that’s what they represent: every value referenced inside the callback should also appear in the inputs array. In the future, a sufficiently advanced compiler could create this array automatically.

useMemo

Pass a “create” function and an array of inputs. useMemo will only recompute the memoized value when one of the inputs has changed. This optimization helps to avoid expensive calculations on every render.

If no array is provided, a new value will be computed whenever a new function instance is passed as the first argument. (With an inline function, on every render.)

Note

The array of inputs is not passed as arguments to the function. Conceptually, though, that’s what they represent: every value referenced inside the function should also appear in the inputs array. In the future, a sufficiently advanced compiler could create this array automatically.

useRef

const refContainer =useRef(initialValue);

useRef returns a mutable ref object whose .current property is initialized to the passed argument (initialValue). The returned object will persist for the full lifetime of the component.

useImperativeMethods

useImperativeMethods(ref, createInstance,[inputs])

useImperativeMethods customizes the instance value that is exposed to parent components when using ref. As always, imperative code using refs should be avoided in most cases. useImperativeMethods should be used with forwardRef:

In this example, a parent component that renders <FancyInput ref={fancyInputRef} /> would be able to call fancyInputRef.current.focus().

useLayoutEffect

The signature is identical to useEffect, but it fires synchronously after all DOM mutations. Use this to read layout from the DOM and synchronously re-render. Updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint.

If you’re migrating code from a class component, useLayoutEffect fires in the same phase as componentDidMount and componentDidUpdate, so if you’re unsure of which effect Hook to use, it’s probably the least risky.